As you probably know... a given color of light can generally, be described by one number (e.g., the frequency of its wave length).

The color white, however, has no associated frequency; it is composed of equal amounts of all the (visible) frequencies of light. Black, of course, is the absence of light. And various shades of grey are generally composed of various intensities of white (as you lower the overall intensity of white, the grey gets darker and approaches black).

The question is: why is light (optically) additive? More specifically, why is it that when we add Red and Green together we get Yellow; Green and Blue together, we get Cyan; Red, Green, and Blue together, we get white?

(Please don't confuse this with the subtractive quality of pigments/paints, where as we add more colors, particularly more of the primary colors red, yellow, and blue, we approach black.)

Could we use three (or more) different colors and achieve the same result?

I believe it's in the eye of the beholder.

Consider 2 beams of monochromatic light of different colors.  The electromagnetic energy of the combination is the sum of 2 sine waves of different frequecies.   This is where the addition comes in.

Inside the retina, the light falling on individual cones stimulates an electrical response in the optic nerve.  Add another wavelength and you add the response of additional cones.

Pigments in paint absorb certain frequencies of light, so adding paint colors is additive with respect to absorption of light, but subtractive with respect to the reflected light (which then may strike someone's retina)

And that's my story.

Posted by Larry on 2004-03-06 15:02:53